Objectives and Background The objective of this research is to increase our understanding of the mechanisms that regulate the expression of B. burgdorferi lipoproteins, a type of molecule that plays a key role in Lyme disease pathogenesis. Last year we reported on the identification of two lipoproteins of Borrelia burgdorferi, P35 and P7.5, whose expression in vitro was transcriptionally regulated in a cell-density- or growth-phase-dependent manner. At low cell density (mid log phase) these proteins were not expressed whereas at higher cell density (late log to stationary phase) they were expressed. Sequencing of the 5' flanking region of the p35 gene revealed a region of dyad symmetry 52 bp upstream of the transcription start site. Results Using an electrophoretic mobility shift assay we have identified a DNA binding protein that binds to a DNA fragment that contains the region of dyad symmetry mentioned above. We have constructed a total DNA library of B. burgdorferi strain B31 in the lambda Zap II bacteriophage vector and are currently screening it in order to clone the DNA binding protein. Future directions To explore the role of the cloned DNA binding protein in the regulation of lipoprotein expression.